I am making a mod for minecraft and I want to know which ID's can be used and which ones are taken by the game allready? I mean which Block/Item ID's (basically a number used to load a block in minecraft)
which ID's are safe to use for my own mod?
According to the wiki Minecraft uses all the block ID's from 0-197. Since unmodded block data is stored in 8 Bit binary, you have 198-256.
If you want your mod to be usable with any others, or in case of future update that adds blocks, You should make it editable in a config file.
Use a high number, and try it. The game will tell you (in a crash report) if you've used an invalid or taken ID.
Once you've found a free ID, you're not done yet though. Use it as the default, but add a config file and read it from there so that end-users can change it. This makes it possible to fix ID conflicts between different mods and makes ID conflicts people run into not your problem.